Stabilized drag bit



1964 H. c. BRIDWELL ETAL 3,120,235

STABILIZED DRAG BIT Filed Jan. 4, 1962 I6 Lil i FIG. I FIG. 2

HAROLD c. BRIDWELL JOHN E. ORTLOFF INVENTORS BY a. Q; I

ATTORNEY United States Patent "ice gen G Esl'l Grtlofi, Tulsa, ()lrla, Researe Company, a

The present invention relates to rotary bits useful for drilling boreholes in the earth and more particularly relates to an improved drag bit provided with abrasion-resistant pads which improve stability, reduce gage wear, and permit more effective circulation of the drilling fluid than has generally been possible with drag bits available in the past.

Difficulties encountered with conventional rotary drag bits in drilling oil wells, gas wells and similar boreholes have limited the application of such bits in recent years. Experience has shown that the use of drag bits frequently results in severe borehole deviation, that drag bit blades often fail rapidly due to wear and abrasion, and that efficient cleaning of the borehole beneath a drag bit is generally a serious problem. Efforts to avoid these difficulties have been only partially successful. Oversize drill collars, stabilizers and similar devices have often been used to hold borehole deviation within acceptable limits but the effectiveness of such devices for controlling deviation in a particular operation is by no means certaiti. The application of improved tungsten carbide and similar hard metal compositions to dra bit blades has increased the resistance of such blades to wear and abrasion but has not prevented fractures at the gage edges due to extremely high impact loadings as the lades strike the borehole wall. The use of jet nozzles and high circulation rates has improved the entrainment of cuttings but has not eliminated the tendency of the cuttings to ball up or accumulate at the face of a drag bit blade. Because of these difficulties, drag bits are not widely used in oil field drhling operations.

It is therefore an object of the present invention to provide an improved rotary drag bit which will permit the drilling of boreholes without the severe deviation problems generally encountered drag bits available in the past. Another object is to provide a drag bit having blades protected against fractures due to impact of the gage edges against the borehole wall. A further object is to provide a drag bit beneath which drilling fluid can be circulated more efficiently in order to prevent the balling-up of cu"- tings adjacent the blade surfaces. Still other objects will become apparent as the invention is described in greater detail hereafter.

in accordance with the invention, it has now been found that many of the difficulties encountered with drag bits available in the past can be avoided by utilizing bits provided with abrasion-resistant pads located on the outer surface of the body between the blades. Tests have shown that such pads effectively reduce the tendency of drag bits to drill oversize holes, decrease the erratic motion of such bits, prevent unequal penetration of the blades into the formation at the bottom of he borehole, preclude high impact loading of the gage edges of the blades, permit the c rculation of drilling fluid at high velocities adjacent the blade surfaces, and assure withdrawal of the worn bit from the borehole without serious damage to the body or nozzles. The pads thus permit better control of borehole deviation, improve the life of the blades, and prevent the balling up of cuttings. The result is a considerably more effective drag bit than has been available in the past.

The improved drag bit of the invention is characterized by the presence of pads on the outer surface of the body between the blades. The pads project outwardly from promotes smoother operation.

Fatented Fe 4, lis -S4 the body to substantially the outer limits of the gage surfaces and extend over from about 25 to about of the body periphery between the blades. Each pad abuts against the rear surface of a blade. Each pad preferably extends vertically from a point above the gage surfaces to a point near the bottom of the body. The pads may be welded in place or may be cast as an integral part of the body. Tungsten carbide, titanium carbide, tantalum carbide or a similar material is used to protect the outer surface of each pad against wear and abrasion.

The exact nature and objects of the invention can best be understood by referring to the following detailed description of a rotary drag bit provided with abrasion resistant pads in accordance with the invention and to the accompanying drawing, in which:

FIGURE 1 is a vertical elevation of a drag bit having abrasion resistant pads extending outwardly from the body between the blades;

FIGURE 2 is a vertical elevation of the bit depicted in FIGURE 1 rotated through an angle of and FIGURE 3 is a bottom View of the bit shown in FIG- URES l and 2.

Turning first to FIGURE 1 of the drawing, it will be noted that the drag bit depicted therein includes a hollow steel body ll of conventional design. To permit attachment of the bit to the lower end of a drill collar or other rotary drill string component, the body contains a standard A.P.l. tool joint box at its upper end. The box is not shown in the drawing. An A.P.I. tool joint pin or other means for connecting the bit to a drill string may be utilized in lieu of a box. Nozzles 12 and 13 are located near the lower end of the body to permit the discharge of drilling fluid against the formation at the bottom of the borehole. Only one nozzle is shown in FIGURE 1. The nozzles will normally be made of tungsten carbide or a similar erosion-resistant material and may be welded to the body or held in place by snap rings or the like. Each nozzle is oriented so that the drilling fluid discharged will impinge against the formation between the center of the hole and the borehole wall at a point a short distance in front of a blade. The nozzles are preferably designed so that the fluid can be discharged at high velocity.

Blades l4- and 315 of steel or other metal are attached to the body of the bit shown in FIGURE 1 and extend downwardly below it in spaced relationship to the nozzles. Each blade is cast to fit closely against the side and bottom of the body and is welded in place. The weld metal is indicated by reference numeral 16. The blades shown are stepped across the bottom and are thus provided with lower edges 17 and 13 which drill the outer portion of the hole during the initial stages of a drilling operation and with upper edges H and 2b which drill the inner portion f the hole. This results in the formation of a short core which assists in centering the bit in the hole and generally The blades may be tapered to reduce the contact area at both the upper and lower steps. As can be seen from FIGURES 2 and 3, the blades are located on opposite sides of a line extending through the axis of the bit. Each blade extends laterially beyond the body, thus providing a gage surface which serves to ream the borehole during a drilling operation. A short filler 21 is welded in place between the blades and the body in order to bond the inner section of the blades to the body more securely.

The faces and gage edges of the blades on the bit shown in the drawing are hard surfaced Wtih a matrix 22 containing particles of tungsten carbide, a tungsten carbide alloy or a similar abrasion-resistant material having a Rockwell A hardness in excess of about 85. A variety of matrix metals may be used, including copper-nickel alloys, copper-nickebtin alloys, copper-nickel-manganese alloys, irou-nickel-manganese alloys, S-lvlonel and the like.

Powdered tungsten carbide may be added to the matrix metal to increase its strength and abrasion resistance. The carbide particles 23 on the faces of the blades are irregularly shaped particles of tungsten carbide ranging between about A and about 7 inch in size. The particles 24 on the gage surfaces are tungsten carbide cubes about A inch along each edge. Hard surfaces containing a matrix and embedded particles of tungsten carbide or a similar hard metal may be produced by powdered metallurgy techniques. A preferred process for the manufacture of such materials is described in co-pending application S.N. 136,308, filed in the name of Harold C. Bridwell and David S. Row-ley on September 6, 196 1. It Will be understood, however, that the invention is not limited to the use of blades hard surfaced in this manner and that pads or inserts of tungsten carbide or a similar hard surfacing material may be bonded to the blade surfaces or embedded in holes in the blades if desired. The blades shown include diamonds 25, about /s carat in size, embedded in the gage surfaces to further increase the abrasion resistance of the bit.

Abrasion-resistant pads 26 and 27 are mounted on the body of the bit between the blades in accordance with the invention. Each pad is cast or machined of steel or similar metal and is hard surfaced wtih a matrix containing tungsten carbide particles or a similar material indicated by reference numerals 28 and 29. In lieu of such a hard surfacing material, inserts of tungsten carbide or the like may be embedded in holes drilled in the outer surface of each pad. .The pads shown are welded in place against the body and blades. Each pad abuts against the rear surface of a blade and extends about the body to a point about midway between the rear surface of the one blade and the face of a following blade. The pads project outwardly to substantially the outer limits of the gage surfaces of the blades. The diameter of the tool through the pads may be from about A; to about inch less than that across the blades if desired but should never be greater than 'the blade diameter. Each pad extends vertically from a point near the bottom of the body to a point above the gage surfaces. The pads thus reduce the annular space bet-ween blades through which drilling fluid may pass upwardly about the bit. The bit shown in the drawing has two blades and is thus referred toas a two-way bit. The invention is applicable to two-way, three-way and four-Way bits.

The bit shown in the drawing may be utilized with a conventional rotary drill string and surface equipment. The bit is connected to the lower end of the drill string and lowered into the borehole in the usual manner. Drilling fluid is circulated downwardly through the string and returned to the annulus between the drill string and the borehole wall. Once circulation has been esablished, drillin iscomrnenced by rotating the string from the surface. During normal operation the bit runs smoothly in the borehole. The large areas presented to the borehole wall by each pad limits the lateral drilling action of the blades on the bit and prevents the drilling of an oversize hole. The pads also limit wear at the gage edges of the blades. Contact between the blades in the bottom of the borehole is more uniform because of the stabilizing effect of the pads and hence life of the bottom hole cutting elements is improved. The more stable bit action thus otbained improves the footage obtained with the bit and reduces stresses of the drill collar, drill pipe and other equipment due to erratic bit motion.

When one of the blades on the bit tends to penetrate more deeply into the formation than does the other, due to the presence of dipping strata at the bottom of the borehole for example, the bit will tend to pivot about the more deeply embedded blade. This normally leads to a sharp impact between the face or gage edge of the opposing blade and the borehole wall. Fracturing of the hard surfacing material on the face or gage edge of the blade may result. The pads on the bit of the invention avoid this. As a bit provided with the pads tends to pivot about one blade, the pad to the rear of that blade contacts the borehole wall. Continued movement after such contact is made necessitates that the bit pivot about the point of contact, rather than about the embedded blade. This dislodges the embedded blade and permits normal rotation of the bit. High impact loadings likely to damage the face and gage surface of the blade are thus avoided.

The placement of pads on the body of the bit between the blades in accordance with the invention improves the circulation of the drilling fluid in addition to promoting smoother bit action. The pads reduce the annular area through which fluid can escape from beneath the bit and hence fluid velocities between the body and the borehole Wall are higher than those normally obtained with conventional bits at equivalent circulation rates. Since each pad is located adjacent the rear face of a blade and extends circumferentially toward the leading face of the following blade, fluid is deflected upwardly by the pads adjacent the leading face of each blade; Tests have shown that the high velocity thus obtained in the critical area in front of each blade where it joins the body prevents the balling-up and accumulation of cuttings normally experienced wtih drag bits. The danger of sticking of the drill string is reduced and higher drilling rates than might otherwise be feasible are obtained.

The pads utilized on thebit of the invention also facilitate withdrawal of the worn bit from the borehole at the proper time. Over a prolonged period the blades on the bit will wear down until little or no bottom hole cutting surface remains. When this occurs, the pads come into contact with the formation at the botttom of the borehole. This results in a large increase in the cross-sectional area in contact with the formation and permits an increase in the weight on the bit without damage to the body or nozzles. Failure of the bit todrill under the increased weight indicates to the driller that the bit should be withdrawn from the hole and replaced or fitted with new blades if the drilling operation is to be continued.

The advantages in utilizing pads mounted on the body of a drag bit between the blades are illustrated by results obtained in two wells about 10,000 feet apart on a lease in southern Texas. The first of these tests was commenced after-the upper section of the well had been drilled to a depth of 1535 feet with roller cone bits and the casing in the upper part of the well had been cemented in place. The drag bit employed in thisfirst test was a 9 /8 inch bit identical to that depicted in the drawing except that two pads mounted on the upper part of the body well above the blades were employed in lieu of pads extending between the blades as shown in the drawing. The bit contained /2 inch welded nozzles and was provided with diamonds at the gage edges of theblades. Eight drill collars 6 inches in outside diameter were connected above the bit. A stabilizer was located between the second and third drill collars in order to help control deviation. This bit was first used to drill from the cement and plugs at the 1535 foot level to a depth. of 2799 feet. At this point it was found that the well has deviated 3% and that deviation was becoming more severe The bit was thereupon removed from the borehole. Examination showed little wear but indicated some damage to the face and gage edge of each blade above the lower cutting surface due to impact between the blade and the wall of the borehole. The pads initially employed were thus obviously not effective for purposes of the invention.

The pads previously used to stabilize the bit were then cut off and the bit was returned to the borehole. Drilling was continued to a depth of 3046 feet. By reducing the weight on the bit to between 5,008 and 10,000 pounds, deviation was gradually reduced to 2 from the maximum of 3% obtained at a depth of 2912 feet. Examination of the bit after the 3046 level had been reached showed severe wear at the gage edges adjacent the body. Wear on the innermost step of the blade and at one side'of the bit shank indicated that the bit had been drilling &- center. Eccentric motion of the bit apparently caused it to drill a large diameter hole and leave a core at the center of the hole.

Tests to determine whether the tendency of the drag bit to drill off-center was due to eccentricity in the bit or sub used to connect it to the lowermost drill collar were carried out. The tests showed that both the drag bit and the sub were true. While these tests were being made, a conventional roller cone bit was used to continue the drilling operation. The cone bit was worn out at a depth of 4730 feet. The drag bit was then returned to the borehole with the drill collars re-arranged so that the two collars previously mounted just above the bit were shifted to the top of the drill collar column. Drilling was again resumed. The performance of the bit in the interval between 4730 and 4746 feet clearly showed that crooked drill collars were not responsible for the bits drilling off-center. A study of the wear pattern on the bit indicated that it had been rotating about a center some distance from the bit axis, thus drilling a large hole and leaving a core in the center of the hole as the bit axis rotated. Measurements of the wear arcs on the bit and the construction of a graphical lay-out indicated that the hole diameter where the wear occurred was between about 11 and about 13 inches and that the core diameter was between 3 and 4 inches. The lay-out also showed that the existence of the core impeded circulation of the drilling fiuid through one nozzle on the bit and had an adverse effect on the cleaning of cuttings from the bottom of the hole.

In order to correct the tendency of the bit to drill offcenter, pads similar to those shown in the drawing were then welded to the bit body adjacent the blades. Each pad extended from a point near the bottom of the body to a point above the upper gage edges of the blades. Each was mounted directly behind a blade so that it occupied about 50% of that portion of the annulus between the rear surface of one blade and the face of the following blade. The pads were initially about /s of an inch under gage radius and were built up to within about a inch of the outer limits of the gage edges by welding tungsten carbide to the outer surfaces of the pads.

The drilling operation was then resumed with the moditied bit. It was found that the pads significantly improved bit performance. Operation was much smoother than it had been before the pads were installed. Better drilling rates were obtained. At a depth of 5586 feet, the bit failed to drill in response to an increase in bit weight. The bit was withdrawn from the borehole and found to be completely worn out. The gage edges of the blades were still square, however, and showed no further damage due to impact against the borehole wall. The borehole itself was not over-sized, indicating that the pads had corrected the tendency to drill over-sized hole. An analysis of the drilling record and comparison of the results obtained with those in other wells in the same area showed that the drag bit had drilled considerably more total footage than the average rock bit in the same area. The average rock bit footage in the interval between 4700 and 5600 feet was found to be 547 feet, based on the records of three different contractors working in the same area. The drag bit drilled about 856 feet after the pads had been added and undoubtedly would have drilled considerably farther if it had not been used previously in the upper section of the hole. The average drilling rate obtained with the rock bits in the same interval was about 17.8 feet per hour. That obtained with the drag bit fitted with pads between the blades was about 16.5 feet per hour.

As pointed out earlier, the second test was carried out in an off-set well located about 10,000 feet from the first well. Geological conditions in the two wells were similar. The bit employed in the second test was a 9% inch bit similar to that used earlier and to that shown in the drawing, except that it was not fitted with pads. Three 8 inch drill collars and sixteen 6% inch drill collars were used in the drill string above the bit. Drilling with the drag bit was commenced at the 1535 foot depth, after roller cone bits had been used to drill the upper part of the hole and casing had been set. The cement and plug at the bottom of the hole were penetrated without difficulty. Drilling was satisfactory to a depth of 3355 feet, at which point difiiculties due to the drilling of a large diameter hole and the cutting of a core were encountered. The action of the bit was very similar to that in the earlier test. The weight of the drill string was reduced and the operation was continued at a very low drilling rate to a depth of 3711 feet. At this point, the bit was withdrawn from the hole and examined. The wear pattern confirmed that the bit had been drilling ofi-center and cutting a core. Some damage to the gage surfaces due to the eccentric drilling action had occurred.

Pads similar to those shown in the drawing were then welded onto the bit body between the blades. Each pad abutted the rear surface of a blade and extended about around the body toward the leading face of the following blade. The diameter through the pads was about A of an inch less than that through the gage surfaces of the blades. With the pads installed, drilling was restuned and continued to a depth of 5,463 feet. No difliculties were encountered. It was found that action of the bit was much smoother than it had been previously and that less pressure was required for cleaning of the borehole than had been necessary earlier. One pump was found to be adequate for the removal of cuttings and the prevention of balling; whereas two pumps had been required prior to the installation of the pads. A comparison of the performance of this bit with that of other 9% inch bits employed by drilling contractors in the same field and at equivalent depths showed that the modified drag bit provided with pads between the blades drilled 99% more footage than the average for all roller cone bits. This extended bit life reduced by at least 1 bit the total number of bits required to drill the test well.

It will be understood that the invention is not limited to the use of the specific hard facing material described or the particular blade configuration shown in the drawing. Tests subsequent to those described above have demonstrated the advantages of using pads on the body between the blades of other drag bits. Such pads tend to stabilize such drag bits, prevent the drilling of oversized boreholes, minimize fractures and excessive wear of blades at the gage edges, permit more effective cleaning of cuttings from beneath the bits, and provide a convenient means for determining when such a bit should be pulled from the borehole. These advantages make the pads attractive for use on drag bits of many different designs.

What is claimed is:

l. A rotary drag bit comprising a hollow body provided with means for connecting said body to the lower end of a drill string; a plurality of blades attached to and depending from said body, said blades extending laterally beyond said body; and a plurality of pads on the outer surface of said body behind said blades, said pads projecting outwardly to substantially the radial limits of said blades but not projecting beyond said blades, said pads extending circumferentially from the rear of said blades over about 25 to about 75 percent of the periphery of said body between said blades, and said pads extending upwardly from points near the bottom of said body to points on said body near the upper limits of the gage edges of said blades.

2. A rotary drag bit comprisin a body member containing a longitudinal fluid passage; said body member including means for attaching said member to the lower end of a drill string; a plurality of blades attached to and depending from said body member, said blades including gage surfaces extending laterally beyond said body member; and a plurality of abrasion-resistant pads mounted on the outer surface of said body member behind said blades, said pads projecting radially adjacent to but not beyond said gage surfaces at the rear of said blades, said pads extending circumferential-1y about said body from the rear of said blades over about 25 to about 75 percent of the body periphery between said blades, and said pads extending from points near the bottom of said body to points above said gage surfaces on said body.

3. A bit as defined by claim 2 wherein said pads extend about said body to points about midway between said blades.

4. A bit as defined by claim 2 wherein said pads are integral With said body.

5. A bit as defined by claim 2 wherein said pads are recessed about /s inch with respect to said gage surfaces of said blades.

6. A rotary drag bit comprising a hollow body me nber having an upper inlet for the admission of drilling fluid and nozzles ior the discharge of said fluid beneath said member, said body member including means for connecting said member to the lower end of a drill string; elongated blades attached to said body member and depending therefrom, said blades havin'g abrasion-resistant gage surfaces extending laterally beyond said body member at spaced intervals about the periphery of said member; and abrasion-resistant ipads mounted on the outer surface of said body member between said gage surfaces of said blades, each of said pads projecting radially to within about A; inch of the outer limits of said gage surfaces each pad extending about said body from the rear surface of one blade about 25 to about-75 percent of the distance to the following blade, each :pad extending upwardly from a point near the bottom of said body to a point on said body above the gage surfaces of said blades,

and the upper surface of each pad tapering upwardly to-' ward said body.

7. A bit as defined by claim 6 wherein each pad extends from the rear surface of one blade to a point about midway between said rear surface and the face of the following blade.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A ROTARY DRAG BIT COMPRISING A HOLLOW BODY PROVIDED WITH MEANS FOR CONNECTING SAID BODY TO THE LOWER END OF A DRILL STRING; A PLURALITY OF BLADES ATTACHED TO AND DEPENDING FROM SAID BODY, SAID BLADES EXTENDING LATERALLY BEYOND SAID BODY; AND A PLURALITY OF PADS ON THE OUTER SURFACE OF SAID BODY BEHIND SAID BLADES, SAID PADS PROJECTING OUTWARDLY TO SUBSTANTIALLY THE RADIAL LIMITS OF SAID BLADES BUT NOT PROJECTING BEYOND SAID BLADES, SAID PADS EXTENDING CIRCUMFERENTIALLY FROM THE REAR OF SAID BLADES OVER ABOUT 25 TO ABOUT 75 PERCENT OF THE PERIPHERY OF SAID BODY BETWEEN SAID BLADES, AND SAID PADS EXTENDING UPWARDLY FROM POINTS NEAR THE BOTTOM OF SAID BODY TO POINTS ON SAID BODY NEAR THE UPPER LIMITS OF THE GAGE EDGES OF SAID BLADES. 